As the Hall-Petch relationship teaches, metal material strength increases with decreasing crystal grain diameter D, and such strength dependency on grain diameter holds even at or near D=50 to 100 nm that means nano-size level crystal grains. Thus, reducing crystal grain diameters down to the ultra-fine, nano-size levels now becomes one of the most important means ever for the reinforcement of metal materials. Some technical journals suggest that reducing D down to ultra-fine sizes of as fine as a few nm causes superplasticity to come out.
There are also some reports that regarding magnetic elements such as iron, cobalt and nickel, in nano-order grain ranges coercive force decreases and soft magnetism improves with decreasing D, which are not found when the crystal grain diameter D is in micron-order ranges.
However, the crystal grain diameter D of most metal materials produced by melting are usually on the order of a few microns to a few tens of microns, and D can hardly be reduced down to the nano-order even by post-treatments. Even with controlled rolling that is an important micro-processing of steel crystal grains, for instance, the lowest possible limit to grain diameters is of the order of at most 4 to 5 μm. In other words, with such ordinary processes it is impossible to obtain materials whose grain diameters are reduced down to the nano-size level.
For instance, intermetallic compounds such as Ni3Al, Co3Ti, Ni3(Si, Ti) and TiAl that provide useful heat-resistant materials and super hard materials, and oxide- and non-oxide based ceramic materials such as Al2O3, ZrO2, TiC, Cr3C2, TiN and TiB2 are all generally less susceptible to plastic processing at normal temperature because of being fragile, and forming processes using super plasticity in relatively high temperature regions become very important.
For the development of superplasticity, however, it is required to reduce their crystal grain diameters down to the nano-size level or an nano-order close thereto. Never until now are there any ultra-fine powders sufficient to meet such forming processes available.
As nitrogen (N) in an amount of, e.g., about 0.9% (by mass) is added to a chromium-nickel type stainless steel having a composition equivalent to that of SUS 304 that is typical austenite stainless steel, the resulting stainless steel having a high nitrogen concentration increases in offset yield strength (yield strength) to about three times as high as that of SUS 304 stainless steel, with no decrease in fracture toughness yet with much more improvements in corrosion resistance in general and pitting corrosion resistance in particular and much more reductions in sensitivity to stress corrosion cracking. Moreover, nitrogen, because of being an extremely strong austenite-stabilization element, is not only capable of superseding expensive nickel with no damage to the above strength properties and corrosion resistance, but also has superior properties such as the effect on holding back process-inducing martensitic transformation under intensive cold processing conditions.
Such effects of N are also true for chromium-manganese type austenite steels. From such considerations, chromium-nickel and chromium-manganese type austenite steels having a high nitrogen concentration have recently attracted considerable attentions as the coming generation of promising new materials.
So far, high-N austenite steels having nitrogen in an amount of up to about 0.1 to 2% (by mass) have been manufactured by melting solidification processes usually in nitrogenous atmospheres, high-temperature solid diffusion sintering processes in nitrogen gas atmospheres, etc. With those processes, however, it is required that the higher the concentration of nitrogen in the end steel, the higher the pressure of nitrogen gas in the atmosphere, offering problems in connection with high-temperature, high-pressure operations and work safety.
Referring here to generally available steel materials inclusive of austenite steel, the finer the crystal grains, the ever higher the effect on strength (hardness) becomes, as is the case with other metals, and high-N austenite steel, too, is now intensively studied for much finer crystal grain diameters. However, it is still very difficult to reduce crystal grains down to the nano-size level; any satisfactory ultra-fine crystal grain material is not achievable as yet, although some high-N austenite steels having a crystal grain structure of the order of a few tens of μm are somehow obtainable.
But then, in high-manganese austenite that attracts great attention as a steel species that could have a dominant role in the coming generation of large-scale technologies (peripheral technologies in linear motor cars, superconduction applied systems, etc.), too, any material having a crystal grain structure of the nano-order is not available as yet, as is the case with the chromium-nickel, and chromium-manganese type austenite steels.